Passive monitoring systems for communications between transmitters in cellular telephone systems are used by operational support system providers to monitor the traffic and sometimes the content of the communications on such transmitters, referred to as user equipment (UE) in the following discussion.
Ideally, monitoring is performed without the active participation of the cellular provider or the UE that is being monitored so that the cellular provider does not need to alter its operation during monitoring. Hence, the monitoring system preferably taps into the data flow between the cellular system and the UE. Monitoring systems for communications in earlier cellular systems tapped a point in the data flow that is no longer available in 4G networks. In addition, the data that is available in 4G networks is in the form of baseband carrier data that encodes the entire set of transmissions between the cellular transmitter and all of the UE in a cell in a manner that can be easily utilized by a radio head transmitter without introducing delays into the transmissions. To monitor any particular transmission, the monitoring system must know the manner in which the communication channel that is available for tapping is organized. This organization can vary from cell to cell in a system, and hence, must be discovered by the monitoring system if the cooperation of the cellular provider is to be avoided.
Remote radio heads and remote radio head controllers use a Common Public Radio Interface (CPRI) that typically goes through a set of initialization stages when first connected together to negotiate a number of important attributes such as line rate, CPRI word length, number of antenna carriers, IQ sample size for downlink and uplink, and type of padding. While systems that monitor the startup phase can be used to provide the needed information on the organization of the data flow, these systems must have a probe in place during the startup phase of the system. Since a passive CPRI probe may be connected to such interfaces at any time after the system has been configured, it cannot be assumed that the probe will observe this initial negotiation phase to determine the needed parameters. Thus a probe that can learn these parameters from the observed byte stream is needed.